Recently, resin materials have been widely used in optical parts such as an optical over-coating agent, a hard coating agent, an antireflection film, an eyeglass lens, an optical fiber, an optical waveguide, and a hologram because resin materials have good workability and high productivity. Resin materials having a high refractive index have been anticipated due to the trend of reducing the sizes and thicknesses of optical parts or from the viewpoint of controlling antireflection property. In particular, there have recently been growing demands for reduction in size and increases in resistivity and brightness in liquid crystal display elements used as a display of a liquid crystal television, a notebook computer, a portable gaming device, a cellular mobile phone, or the like. To meet these demands, it is essential to increase the refractive index of prism sheets.
Manufacture of formed materials such as a prism sheet requires optical materials having a high refractive index and a low viscosity. However, existing resin materials have a problem in that an increase in refractive index results in an increase in viscosity and leads to crystallization.
There has recently been proposed, as an optical material having a high refractive index, a compound having a fluorene skeleton to provide a material having a high refractive index. Known examples of such a compound include a difunctional compound in which an acryloyl group is bonded to a fluorene skeleton through an alkyleneoxy group (see PTLs 1 to 3); and a compound produced by reacting a diglycidyl ether having a fluorene skeleton with acrylic acid or methacrylic acid (see PTL 4). These compounds have received attention since they have high heat resistance and a high refractive index. However, these fluorene derivatives have the following problem. These fluorene derivatives are generally solids or liquids having a high viscosity of several tens of Pa·s or more at ordinary temperature. Therefore, when these fluorene derivatives are used for formed materials such as a prism sheet, dilution with a large amount of reactive diluent or the like is required to achieve an adequate viscosity. This reduces the refractive index of the resulting cured product.
Examples of currently used reactive diluents include phenylthioethyl acrylate (PTEA), o-phenylphenoxyethyl acrylate (OPPEA), and naphthylthioethyl acrylate (NTEA). PTEA, despite having a high refractive index and a low viscosity, is not preferable because of its poor lightfastness and strong odor due to sulfur contained therein, which results in poor workability. NTEA is unsatisfactory in terms of lightfastness since it has a naphthalene ring. Thus, currently, OPPEA is often selected.
A combination of a difunctional (meth)acrylate compound having a fluorene skeleton and OPPEA has been proposed as a composition for use in optical materials (see PTL 5). However, OPPEA, despite being a reactive diluent having a relatively high refractive index of 1.576, fails to reduce the viscosity of the difunctional (meth)acrylate compound having a fluorene skeleton sufficiently when used alone. Therefore, an example described in PTL 5 employs a reactive diluent as a third component. Thus, a reactive diluent that has a high refractive index and that allows the resulting composition to have a sufficiently low viscosity for formed material use has been anticipated.